1. Field of the Invention
The present invention relates to an electro-optic probe adapted to couple an electrical field generated by a measurement signal with an electro-optic crystal, when a beam of light is incident onto this electro-optic crystal, the waveform of the measurement signal can be observed by the polarization state of the incident light. More particularly, the present invention relates to an electro-optic probe having an improved optical system therein.
This application is based on Japanese Patent Application No. Hei 11-206876, and No. Hei 11-275389 the content of which is incorporated herein by reference.
2. Description of the Related Art
It is possible to couple an electrical field generated by a measurement signal with an electro-optic crystal, allow a laser beam to be incident into this electro-optic crystal, and observe the waveform of the measurement signal by the polarization state of the laser beam. It is possible to convert the laser beam to a pulse-shaped laser beam, and make observations with an extremely high time resolution when sampling the measurement signal. A device that does this is an electro-optic sampling oscilloscope that uses the electro-optic probe exploiting this phenomenon.
When this electro-optic sampling oscilloscope (hereinbelow, referred to as an xe2x80x9cEOS oscilloscopexe2x80x9d) is compared to a conventional sampling oscilloscope using an electrical probe, the following characteristics are notable:
1. It is easy to measure the signal because a ground wire is unnecessary.
2. Because the metal pin at the tip of the electro-optic probe is insulated from the circuit system, it is possible to realize a high input impedance. As a result of this, there is almost no degradation of the state of the measurement signal.
3. By using an optical pulse, broadband measurement up to the GHz order is possible.
The structure of an electro-optic probe in the conventional technology will be explained using FIG. 7. In FIG. 7, reference numeral 1 denotes a probe head composed of an insulator, and a metallic pin la is fitted in the center thereof. Reference numeral 2 denotes an electro-optic element, which includes a reflective coating 2a provided on an end surface on the metallic pin 1a side, and which is in contact with the metallic pin 1a. Reference numerals 3 and 9 denote collimator lenses. Reference numeral 5 denotes a 1/4 wavelength plate. Reference numerals 6 and 8 denote polarizing beam splitters. Reference numeral 7 denotes a Faraday element. Reference numeral 10 denotes a laser diode provided to emit a laser beam in response to a pulse signal outputted from an EOS oscilloscope body (not shown).
Reference numerals 11 and 13 denote condensing lenses, and reference numerals 12 and 14 denote photodiodes provided to convert the input laser beam into an electrical signal and output the signal to the EOS oscilloscope body. Reference numeral 15 denotes a probe body. Reference numeral 17 denotes an isolator comprising the 1/4 wave plate 5, the two polarizing beam splitters 6 and 8, and the Faraday element 7, for passing light output from the laser diode 10 and isolating the light reflected by the reflective coating 2a. 
Next, referring to FIG. 7, the optical path of the laser light emitted from the laser diode 10 will be explained. In FIG. 7, a reference code A denotes the optical path of the laser beam.
First, the laser beam emitted from the laser diode 10 is converted by the collimator lens 9 into a parallel beam. This parallel beam travels straight through the polarized beam splitter 8, the Faraday element 7 and the polarized beam splitter 6, and then passes through the 1/4 wavelength plate 5 to be condensed by the collimator lens 3, and is incident onto the electro-optic element 2. The incident light is reflected by the reflective coating 2a formed on the end surface of the electro-optic element 2 on the side facing the metallic pin 1a. 
Then, the reflected laser beam is again converted into a parallel beam by the collimator lens 3, and travels through the 1/4 wavelength plate 5. A part of the laser beam is reflected by polarized beam splitter 6, condensed by the condensing lens 11, and is incident onto the photodiode 12. The laser beam that has traveled through the polarized beam splitter 6 is reflected by the polarized beam splitter 8, condensed by the condensing lens 13, and is incident onto the photodiode 14.
The 1/4 wavelength plate 5 serves to adjust the laser beams incident on the photodiodes 12 and 13 such that intensities thereof are uniform.
Next, using the electro-optic probe shown in FIG. 7, the procedure for measuring the measurement signal is explained.
When the metallic pin 1a is placed in contact with a measurement point, at the electro-optic element 2, the electrical field is propagated to the electro-optic element 2 by a voltage applied to the metallic pin 1a, resulting in the phenomenon that the refractive index is changed due to the Pockels effect. Accordingly, the laser beam emitted from the laser diode 10 is incident onto the electro-optic element 2 and, when the laser beam is propagated along the electro-optic element 2, the polarization state of the beam changes. Then, the laser beam having this changed polarization state is reflected by the reflective coating 2a, condensed and is incident onto the photodiodes 12 and 14, and converted into an electrical signal.
The change in polarization caused by the electro-optic element 2, accompanying a change in voltage of the measurement point, results in a difference in the output from the photodiodes 12 and 14, and by detecting this difference it is possible to measure the electrical signal applied to the metal pin 1a. 
In the above-described electro-optic probe, the electrical signals obtained from the photodiodes 12 and 14 are input into the EOS oscilloscope, and processed. Instead, however, it is possible to measure the signals by connecting a conventional measuring device such as a real time oscilloscope to the photodiodes 12 and 14 via a dedicated controller. In this way, it is possible to carry out broadband measurement simply by using the electro-optic probe.
The electro-optic probe as shown in FIG. 7 uses internal optical components. These optical components have the characteristic that when the temperatures of the components changes, their refractive indexes change. The change in the refractive index directly leads to a measurement error. It is therefore preferred that the optical components are maintained at the same temperature as that at the time of probe calibration.
However, since an operator must hold the electro-optic probe shown in FIG. 7 for use, it is difficult to maintain constant the temperature at which the probe is used. The change in an ambient temperature also brings about changes in the temperatures of the components constituting the probe, which in turn reduces the accuracy of measurement.
In consideration of the above-described situation, it is an object of the present invention to provide an electro-optic probe capable of maintaining a constant accuracy of measurement even if there is a change in temperature.
In order to achieve this object, the electro-optic probe of the invention comprises:
a laser diode provided to emit a laser beam based on a control signal of an oscilloscope body;
a collimator lens provided to convert the laser beam into a parallel beam;
an electro-optic element having on an end face thereof a reflective coating, with optical characteristics which are changed by propagating an electrical field via a metal pin provided at the end face on the reflective coating side;
an isolator provided between the collimator lens and the electro-optic element, and adapted to transmit the laser beam emitted from the laser diode in order to separate a light reflected by the reflective coating from the laser beam; and
a photodiode provided to convert the reflected light separated by the isolator into an electrical signal;
wherein the electro-optic probe includes:
a temperature detection section arranged to be in contact with optical components constituting the isolator, and to detect the temperature of the optical components and output the result of the detection;
a temperature adjustment section arranged to be in contact with the laser diode, and to adjust the temperature of the laser diode according to the result of the temperature detection; and
a temperature control unit provided to control the temperature adjustment section based on the result of the temperature detection.
As described above, according to the invention, the temperature detection section is provided so as to be in contact with the optical components constituting the isolator, and the temperature adjustment section is provided to adjust the temperature of the laser diode based on the detection result of the temperature detection section. Therefore, it is possible to maintain constant S/N all the time, and it is also possible to prevent the accuracy of measurement from decreasing due to a temperature change.
The reference numerals appended in the claims do not limit the interpretation of the claims.